Directional couplers are devices that couple a portion of a signal's power in a transmission line to a port that is often called the coupled port. Also, directional couplers typically include an input port and a transmitted port associated with the transmission line, and an isolated port that corresponds to the coupled port.
Various characteristics are used in evaluating the performance of couplers. One of these characteristics is the coupling factor, which is the ratio of signal levels between the input port and the coupled port. Another characteristic is isolation, which is a ratio of signal levels between the input port and the isolated port. A further characteristic, directivity, is a ratio of signal levels between the coupled port and the isolated port. Alternatively, directivity may be expressed as a ratio between the isolation and the coupling factor.
Generally, high isolation and high directivity values are desirable. In contrast, low values typically indicate deficient performance. For instance, as isolation decreases, the amount of power that is “leaked” from the input to the isolated port increases. Also, as directivity decreases, small mismatches on the transmission line can cause variations in coupled power levels.
Existing coupler design techniques result in a prohibitive trade-off between size and performance. For instance, typical couplers providing suitable performance characteristics are large in size (e.g., on the order of a quarter wavelength). Thus, these couplers are too large for applications, such as cellular handsets. Also, despite being somewhat suitable, such large couplers have excessive path lengths, which can cause unwanted losses and undesirable system efficiency.